Groundwater electrical conductivity and soil radon gas monitoring for earthquake precursory studies in Koyna, India

► It is the first hydrochemical precursory study in the Koyna region, India. ► Discrete conductivity measurements indicated progressive increase for 4 years. ► Strong precursory EC change observed 40 h before the M 5.1 earthquake. ► Precursory increase of soil Rn gas 20 days earlier than earthquakes M 4.7 & 5.1. ► On-line monitoring of these parameters may help in earthquake forecast.